Birds
played a major role in creating awareness of pollution
problems. Indeed, many people consider the modern
environmental movement to have started with the publication
in 1962 of Rachel Carson's classic Silent Spring, which
described the results of the misuse of DDT and other
pesticides. In the fable that began that volume, she wrote:
"It was a spring without voices. On the mornings that had
once throbbed with the dawn chorus of robins, catbirds,
doves, jays, wrens, and scores of other bird voices there
was now no sound; only silence lay over the fields and woods
and marsh." Silent Spring was heavily attacked by the
pesticide industry and by narrowly trained entomologists,
but its scientific foundation has stood the test of time.
Misuse of pesticides is now widely recognized to threaten
not only bird communities but human communities as
well.

The potentially lethal
impact of DDT on birds was first noted in the late 1950s
when spraying to control the beetles that carry Dutch elm
disease led to a slaughter of robins in Michigan and
elsewhere. Researchers discovered that earthworms were
accumulating the persistent pesticide and that the robins
eating them were being poisoned. Other birds fell victim,
too. Gradually, thanks in no small part to Carson's book,
gigantic "broadcast spray" programs were brought under
control.

But DDT, its breakdown
products, and the other chlorinated hydrocarbon pesticides
(and nonpesticide chlorinated hydrocarbons such as PCBs)
posed a more insidious threat to birds. Because these
poisons are persistent they tend to concentrate as they move
through the feeding sequences in communities that ecologists
call "food chains." For example, in most marine communities,
the living weight (biomass) of fish-eating birds is less
than that of the fishes they eat. However, because
chlorinated hydrocarbons accumulate in fatty tissues, when a
ton of contaminated fishes is turned into 200 pounds of
seabirds, most of the DDT from the numerous fishes ends up
in a relatively few birds. As a result, the birds have a
higher level of contamination per pound than the fishes. If
Peregrine Falcons feed on the seabirds, the concentration
becomes higher still. With several concentrating steps in
the food chain below the level of fishes (for instance, tiny
aquatic plants crustacea small fishes), very slight
environmental contamination can be turned into a heavy
pesticide load in birds at the top of the food chain. In one
Long Island estuary, concentrations of less than a tenth of
a part per million (PPM) of DDT in aquatic plants and
plankton resulted in concentrations of 3-25 PPM in gulls,
terns, cormorants, mergansers, herons, and
ospreys.

"Bioconcentration" of
pesticides in birds high on food chains occurs not only
because there is usually reduced biomass at each step in
those chains, but also because predatory birds tend to live
a long time. They may take in only a little DDT per day, but
they keep most of what they get, and they live many
days.

The insidious aspect of this
phenomenon is that large concentrations of chlorinated
hydrocarbons do not usually kill the bird outright. Rather,
DDT and its relatives alter the bird's calcium metabolism in
a way that results in thin eggshells. Instead of eggs,
heavily DDT-infested Brown Pelicans and Bald Eagles tend to
find omelets in their nests, since the eggshells are unable
to support the weight of the incubating bird.

Shell-thinning resulted in
the decimation of the Brown Pelican populations in much of
North America and the extermination the Peregrine Falcon in
the eastern United States and southeastern Canada.
Shell-thinning caused lesser declines in populations of
Golden and Bald Eagles and White Pelicans, among others.
Similar declines took place in the British Isles.
Fortunately, the cause of the breeding failures was
identified in time, and the use of DDT was banned almost
totally in the United States in 1972.

The reduced bird populations
started to recover quickly thereafter, with species as
different as ospreys and robins returning to the pre-DDT
levels of breeding success in a decade or less. Furthermore,
attempts to reestablish the peregrine in the eastern United
States using captive-reared birds show considerable signs of
success. Brown Pelican populations have now recovered to the
extent that the species no longer warrants endangered status
except in California. The banning of DDT has helped to
create other pesticide problems, however. The newer
organophosphate pesticides that to a degree have replaced
organochlorines, such as parathion and TEPP (tetraethyl
pyrophosphate), are less persistent so they do not
accumulate in food chains. They are, nonetheless, highly
toxic. Parathion applied to winter wheat, for instance,
killed some 1,600 waterfowl, mostly Canada Geese, in the
Texas panhandle in 1981.

Unfortunately, however, DDT
has recently started to become more common in the
environment again; its concentration in the tissues of
starlings in Arizona and New Mexico, for example, has been
increasing. While the source of that DDT is disputed, what
is certain is that DDT has been shown to be present as a
contaminant in the widely used toxin dicofol (a key
ingredient in, among others, the pesticide Kelthane).
Dicofol is a chemical formed by adding single oxygen atoms
to DDT molecules. Unhappily, not all the DDT gets
oxygenated, so that sometimes dicofol is contaminated with
as much as 15 percent DDT

Overall, the 2.5 million
pounds of dicofol used annually in pesticides contain about
250 thousand pounds of DDT. In addition, little is known
about the breakdown products of dicofol itself, which may
include DDE, a breakdown product of DDT identified as the
major cause of reproductive failure in several bird species.
Finally, DDT itself may still be in use illegally in some
areas of the United States, and migratory birds such as the
Black-crowned Night-Heron may be picking up DDT in their
tropical wintering grounds (where DDT application is still
permitted). Unhappily tropical countries are becoming
dumping grounds for unsafe pesticides that are now banned in
the United States. As the end of the century approaches, the
once hopeful trend may be reversing, so that DDT and other
pesticides continue to hang as a heavy shadow over many bird
populations.